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Fundamental physics, as we've seen, finds itself in a difficult situation. Nothing unexpected has turned up at the Large Hadron Collider. We have phenomena like dark matter and dark energy that are defying explanation. And some of the most exciting ideas that theoreticians are coming up with have steadfastly refused to submit to any form of experimental testing.

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One possible route out of this mess is to focus on some of the oddities in the data that we already have. For example, there are a few measurements that seem to show particle behavior that's inconsistent with physics' Standard Model. And there are other cases where two different routes to the same measurement give different results, a possible sign that some new physics is influencing one experimental approach but not another. But before we pursue these oddities, the first step is to confirm that something unexpected is really happening.

This is exactly the situation we have with the decay of neutrons. We have two different ways of measuring the neutron's half-life, and the values they produce disagree by an appreciable amount. To find out whether this disagreement is real, however, we have to increase the precision of the measurements. And that's precisely what a large US-Russian collaboration has done.

Unstable

Neutrons are probably best known for being a chargeless component of the nucleus of all atoms other than hydrogen. In that context, they can be extremely stable—you probably noted the fact that your body wasn't decaying around you. But pull neutrons out of that context, and they become very unhappy. They'll decay into a proton, an electron, and a neutrino. That decay has a half life—the time it would take half the neutrons in a large sample to decay—of a bit under 15 minutes. But just how much less isn't clear.

That's not for lack of measurements. We have plenty of them, many with error bars of less than three seconds. The problem is that these measurements systematically disagree.

Neutrons have no charge, so they're a bit difficult to control, and they tend to undergo reactions with atoms that they happen to bump into. This makes them a challenge to track, but scientists have settled on two methods. One is to produce a beam of neutrons and watch the beam for signs of the decay products. This produces a value for the half-life of 887.7 ± 2.2 seconds. A second method is to try to store the neutrons for a length of time and see how many of them decay. Annoyingly, this produces a value of 878.5 ± 0.8. Those are more than nine seconds apart, a difference of about four standard deviations.

Getting that to the point where the difference is five standard deviations—a value that physics accepts as indicating an effect is real—requires cutting down on those errors. And that's exactly what the new US-Russian work aims to do.

Neutrons in a bottle

This experiment relies on storing a bunch of neutrons in a container. By knowing how many you put in and measuring them some time later, you can figure out how many of them decayed in that time. Get enough of these measurements and you can figure out what the half-life is.

This method may be conceptually simple, but implementing it is another matter entirely. With no charge, neutrons are difficult to control, and they can easily bump into the walls of the container or pick up enough energy to go flying out of it entirely. Plus you have to know how many are there.

To manage this, the researchers start with a beam of slow-moving neutrons. Part of that beam is diverted to a detector, which registers how many neutrons hit it; this gives us a measure of how many neutrons are left in the beam. The remaining neutrons are then dumped into a container, at which point the challenge of keeping them in the container starts.

First, there's keeping them from hitting the walls. Neutrons may not have charge, but they do have spin, allowing them to be influenced by a magnetic field. The container uses an external magnetic field to align the spin of all the neutrons, and magnets lining the walls of the container gently repel the particles, keeping them from hitting the walls. The top of the container is kept open, but the neutrons will typically end up circulating near the bottom of the container due to gravity.

A few of the particles might have high enough energies to escape the container against the pull of gravity, but the researchers use what they call a "cleaner" to handle them. The cleaner is simply a piece of plastic that gets inserted near the top of the container. Any neutrons with enough energy to escape quickly run into it and are removed from the experiment. The container is also asymmetric, which causes other high-energy neutrons to bump up against the magnets and bounce up to the cleaner.

When the researchers were ready to perform a measurement, they simply lowered a detector into the middle of the trap, at which point the neutrons promptly ran into it.

Still disagreeable

The key thing about this setup is that it makes measuring all sources of neutron loss during the experiment relatively easy. By counting the losses, the researchers were able to better estimate the population of neutrons that were available to decay, and therefore they got a better measurement of how many did. Six hundred and sixty-four measurements later, they calculated the half-life.

In an ideal world, the slightly different measurement technique and higher precision of the measurement would get rid of the discrepancy. In case you hadn't noticed, however, our world is far from ideal. The new value, 877 ± 0.7 seconds, actually makes the disagreement a tiny bit worse. Those willing to scroll up to the top of the article would also notice that the ± 0.7 seconds isn't a whole lot better than the previous precision record, ± 0.8 seconds.

But the authors say that this uncertainty is dominated by statistics, not noise, which means it will go down with further measurements. They expect it should simply be a matter of doing more measurements to reduce the uncertainty below ± 0.5 seconds.

Given the described complexity of keeping neutrons, how many different experiments it is possible to make to measure the decay? If I understand correctly, there are two groups of experiments - with the beam and with the "bottle". How slow we can make neutrons go in the beam? Is there a chance to find a threshold between two behaviors? Is this halftime related to the speed? If we take bottle experiment and then measure the halftime only of those neutrons that achieved speed threshold via statistics, do they behave as if they were "bottle neutrons" or "beam neutrons"?

Thanks for the article! It gives me a place I can ask questions I've had regarding neutron decay for some time, but no one to ask!!

1) What does the Standard Model predict for neutron decay? If it can't offer a prediction, why not?

2) Why is the neutron unstable in isolation, but not in a nucleus? What force (or lack there of) is responsible?

3) Why are neutrons stable in neutron stars where they "only" have other neutrons to associate with?

4) Given #3, is it possible that the presence of other neutrons effects the decay rate of a neutron?

5) Given: neutron is 1 up and 2 down quarks; proton is 2 up and 1 down quark; quarks and leptons are two different fundamental particles. Then, when a hydrogen atom of one proton and one electron is crushed in the formation of a neutron star to form a neutron, how does an up quark and an electron combine to form a down quark? That is, how can you transmute a lepton and a quark into a different quark, given that both are already fundamental particles? (The “logical” implication here is that both leptons and quarks must consist of an even more fundamental “particle” that accounts for such transformations. N.B. I never said that quantum mechanics is logical.) The same question in reverse: when a neutron decays, how does a quark transmute into a different quark and a lepton?

Thanks for any insight! These questions have been bugging me for years!!!

Now I'm sure people have accounted for this, but the neutrons in the neutron beam do not happen to move at 2% of the speed of light relative to the lab, do they? Time dilation would then account for the discrepancy.

The earlier definition and the new definition barely stay within the error bars of each other. Only 877.7 is true for both 878.5±0.8 and 877.0±0.7. It makes me wonder if there is a difference between experiments done "in the same way."

The earlier definition and the new definition barely stay within the error bars of each other. Only 877.7 is true for both 878.5±0.8 and 877.0±0.7. It makes me wonder if there is a difference between experiments done "in the same way."

Remember error bars aren't hard limits. They are (typically) the standard deviation of the distribution of the predicted answer based on the measurements. This technically means all the values of the first experiment are within the second's results, with varying degrees of probability density.

A possible explanation to the discrepant result not related in the post is based on the fact that the beam experiments count the protons (as decay product of the neutrons), while the bottle experiments count the remaining neutrons. If neutrons would decay sometimes as other particles, say dark matter particle, the counted protons in beams would be rarer, so the decay time would look longer.

A possible explanation to the discrepant result not related in the post is based on the fact that the beam experiments count the protons (as decay product of the neutrons), while the bottle experiments count the remaining neutrons. If neutrons would decay sometimes as other particles, say dark matter particle, the counted protons in beams would be rarer, so the decay time would look longer.

There are times that the produced proton and electron combine into a hydrogen atom (about four per million).

It seems probable that there could be other modes of decay that would impact the result depending on experiment.

A possible explanation to the discrepant result not related in the post is based on the fact that the beam experiments count the protons (as decay product of the neutrons), while the bottle experiments count the remaining neutrons. If neutrons would decay sometimes as other particles, say dark matter particle, the counted protons in beams would be rarer, so the decay time would look longer.

This would give dark matter a fairly strong coupling to the weak force, which may or may not already have been ruled out.

Thanks for the article! It gives me a place I can ask questions I've had regarding neutron decay for some time, but no one to ask!!

1) What does the Standard Model predict for neutron decay? If it can't offer a prediction, why not?

2) Why is the neutron unstable in isolation, but not in a nucleus? What force (or lack there of) is responsible?

3) Why are neutrons stable in neutron stars where they "only" have other neutrons to associate with?

4) Given #3, is it possible that the presence of other neutrons effects the decay rate of a neutron?

5) Given: neutron is 1 up and 2 down quarks; proton is 2 up and 1 down quark; quarks and leptons are two different fundamental particles. Then, when a hydrogen atom of one proton and one electron is crushed in the formation of a neutron star to form a neutron, how does an up quark and an electron combine to form a down quark? That is, how can you transmute a lepton and a quark into a different quark, given that both are already fundamental particles? (The “logical” implication here is that both leptons and quarks must consist of an even more fundamental “particle” that accounts for such transformations. N.B. I never said that quantum mechanics is logical.) The same question in reverse: when a neutron decays, how does a quark transmute into a different quark and a lepton?

Thanks for any insight! These questions have been bugging me for years!!!

Here's my understanding, for what it's worth:

1) We don't know enough about the various constants involved to predict it. The measurement helps us learn more about the standard model, and possible issues with it.

2) Because for a neutron to decay, it must transition to a net lower energy state. This is easy in isolation but in the context of a nucleus you have to worry about what energy state the resultant proton ends up in (thanks Pauli), and there may not be a low enough state available, depending on the atomic structure. The "force" is conservation of energy and the Pauli Exclusion Principle.

3) Again, conservation of energy plus Pauli. There is no free energy states for the resultant proton to transition to.

4) Sort of. Neutrons affect the proton's energy states by separating them from each other.

5) The weak force is all about transmuting one quark to another. Quarks are not conserved in the way you're assuming. A virtual W-particle enacts the change from up to down. Look at the "beta decay" article on wikipedia for a better description.

Given the described complexity of keeping neutrons, how many different experiments it is possible to make to measure the decay? If I understand correctly, there are two groups of experiments - with the beam and with the "bottle". How slow we can make neutrons go in the beam? Is there a chance to find a threshold between two behaviors? Is this halftime related to the speed? If we take bottle experiment and then measure the halftime only of those neutrons that achieved speed threshold via statistics, do they behave as if they were "bottle neutrons" or "beam neutrons"?

I assume the relativistic effect of the neutrons traveling in the beam is accounted for in the measurement. If neutron half-life changes due to speed by other than the amount indicated by time dilation, that's a divergence from GR, so that's a huge deal.

Thanks for the article! It gives me a place I can ask questions I've had regarding neutron decay for some time, but no one to ask!!

1) What does the Standard Model predict for neutron decay? If it can't offer a prediction, why not?

2) Why is the neutron unstable in isolation, but not in a nucleus? What force (or lack there of) is responsible?

3) Why are neutrons stable in neutron stars where they "only" have other neutrons to associate with?

4) Given #3, is it possible that the presence of other neutrons effects the decay rate of a neutron?

5) Given: neutron is 1 up and 2 down quarks; proton is 2 up and 1 down quark; quarks and leptons are two different fundamental particles. Then, when a hydrogen atom of one proton and one electron is crushed in the formation of a neutron star to form a neutron, how does an up quark and an electron combine to form a down quark? That is, how can you transmute a lepton and a quark into a different quark, given that both are already fundamental particles? (The “logical” implication here is that both leptons and quarks must consist of an even more fundamental “particle” that accounts for such transformations. N.B. I never said that quantum mechanics is logical.) The same question in reverse: when a neutron decays, how does a quark transmute into a different quark and a lepton?

Thanks for any insight! These questions have been bugging me for years!!!

I can't give you a complete answer, but I'll try to answer some of your questions.

Given the described complexity of keeping neutrons, how many different experiments it is possible to make to measure the decay? If I understand correctly, there are two groups of experiments - with the beam and with the "bottle". How slow we can make neutrons go in the beam? Is there a chance to find a threshold between two behaviors? Is this halftime related to the speed? If we take bottle experiment and then measure the halftime only of those neutrons that achieved speed threshold via statistics, do they behave as if they were "bottle neutrons" or "beam neutrons"?

I assume the relativistic effect of the neutrons traveling in the beam. If neutron half-life changes by other than the amount indicated by time dilation, that's a divergence from GR, so that's a huge deal.

It has me wondering how much the individual neutrons deviate from the average speed as well.

Even if they know the average, if some are faster and some are slower it will skew towards a longer decay time, as time dilation is not linear.

How fast were the neutrons travelling in the beam experiment? My intuition is the decay rate could would be slower as observed by us for neutron travelling at close to speed of light due to time dilation, and indeed the decay rate is slower in the beam experiment quoted compared to the stored neutron numbers.

Given the described complexity of keeping neutrons, how many different experiments it is possible to make to measure the decay? If I understand correctly, there are two groups of experiments - with the beam and with the "bottle". How slow we can make neutrons go in the beam? Is there a chance to find a threshold between two behaviors? Is this halftime related to the speed? If we take bottle experiment and then measure the halftime only of those neutrons that achieved speed threshold via statistics, do they behave as if they were "bottle neutrons" or "beam neutrons"?

I assume the relativistic effect of the neutrons traveling in the beam. If neutron half-life changes by other than the amount indicated by time dilation, that's a divergence from GR, so that's a huge deal.

The relativistic effect in decay was certainly taken into account - hey, I learned about muons halftime changes due to particles' speed in high school. There has to be some more subtle difference. As someone explained here the main difference in two experiments is how they measure it. It is always measured indirectly. They have certainly tried to take into account everything, but some small but important detail is missing.

Thanks for the article! It gives me a place I can ask questions I've had regarding neutron decay for some time, but no one to ask!!

1) What does the Standard Model predict for neutron decay? If it can't offer a prediction, why not?

2) Why is the neutron unstable in isolation, but not in a nucleus? What force (or lack there of) is responsible?

3) Why are neutrons stable in neutron stars where they "only" have other neutrons to associate with?

4) Given #3, is it possible that the presence of other neutrons effects the decay rate of a neutron?

5) Given: neutron is 1 up and 2 down quarks; proton is 2 up and 1 down quark; quarks and leptons are two different fundamental particles. Then, when a hydrogen atom of one proton and one electron is crushed in the formation of a neutron star to form a neutron, how does an up quark and an electron combine to form a down quark? That is, how can you transmute a lepton and a quark into a different quark, given that both are already fundamental particles? (The “logical” implication here is that both leptons and quarks must consist of an even more fundamental “particle” that accounts for such transformations. N.B. I never said that quantum mechanics is logical.) The same question in reverse: when a neutron decays, how does a quark transmute into a different quark and a lepton?

Thanks for any insight! These questions have been bugging me for years!!!

1) As far as I understand, a theoretical prediction would have greater uncertainty than an experimental measurement in this case due to the difficulty in QCD computations. Baryon dynamics are difficult to compute. Just recently there was a significant correction to cross-section limits in dark matter detectors due to an update to the modelling of nuclei collisions. Don't remember the details though.

I am not too sure about this though so maybe I'm wrong. I'm not a nuclear physics guy.

3. Same reason as 2. Neutron stars are mostly degenerate, which means the neutrons have taken up almost all of the possible low energy eigenstates. Similarly, other species in a neutron star should be degenerate, and would exclude decay by the same mechanism as above. This is a simplified view because there's actually structure and dynamics in neutron stats.

4. There's no evidence for that. (though as previously mentioned neutrons can have indirect effects on each other by affecting other particles instead.)

5. Conservation of leptop number is enforced by the neutrino tha would be produced in such a process. There's no such implication as you claim. Fundamental and indestructible are different things. Particles aren't conserved. Flavour numbers are, but there are mechanisms to conserve the numbers while allowing for such interactions like I just mentioned.

A possible explanation to the discrepant result not related in the post is based on the fact that the beam experiments count the protons (as decay product of the neutrons), while the bottle experiments count the remaining neutrons. If neutrons would decay sometimes as other particles, say dark matter particle, the counted protons in beams would be rarer, so the decay time would look longer.

This would give dark matter a fairly strong coupling to the weak force, which may or may not already have been ruled out.

It is kinda ruled out by direction detection experiments though it might be possible to construct a theory to get around that. I don't know how one would construct a theory with an interaction that's suppressed at high energies (eg. Detector interactions) while not at low energies (decay)

No, physicists aren't stupid. They know how relativity works. Yes, the simple explanations have been considered. It could definitely still be some systemic error in measurement but that would have to be pretty well hidden. And no, your pet theories with no associated calculations and without a decade or so of physics education and training backing them are largely worthless.

No, physicists aren't stupid. They know how relativity works. Yes, the simple explanations have been considered. It could definitely still be some systemic error in measurement but that would have to be pretty well hidden. And no, your pet theories with no associated calculations and without a decade or so of physics education and training backing them are largely worthless.

You can hardly expect everyone commenting to have a deep background on every subject.

Besides, experience tells me that when you are talking about a bunch of people with a decade of higher education, you most definitely want to double check for simple and obvious causes when there is an issue.

Is it possible that the presence of protons, electrons and neutrinos from the decay influences that decay. In the beam those decay products should have less interference than in a closed vessel.

For the measurements wouldn't it be better to measure the decay products as well as the neutrons, this should give you a smaller margin of error. And as mentioned before, both experiments should be measuring the same thing. If the beam is measuring the decay products the closed vessel should do the same and visa versa. It would be interesting if the half times calculated using the amount of neutrons left would be the same in either experiment but different from the half time calculated using the amount of decay products formed.

It's enough to make me wonder if the decay rate differs depending on the spin, or some other measurable property.

Yeah, except for the fact that those measurements have also been made. It could conceivably be due to one experiment using confined neutrons and the other using free neutrons, but it isn't obvious what the physical cause would be.

No, physicists aren't stupid. They know how relativity works. Yes, the simple explanations have been considered. It could definitely still be some systemic error in measurement but that would have to be pretty well hidden. And no, your pet theories with no associated calculations and without a decade or so of physics education and training backing them are largely worthless.

I find condescension and impatience to much less desirable traits than a layman's interest to engage on a technical subject.

" Those are more than nine seconds apart, a difference of about four standard deviations.

Getting that to the point where the difference is five standard deviations—a value that physics accepts as indicating an effect is real"

No.

No no no.

That means you want to make the difference worse, not better. Ideally, you would have two overlapping ranges and that means less than one standard deviation.

The five sigma figure applies to a totally different type of experiment. Suppose you think you have identified a new particle. Five sigma is a measure of the probability that your observation is not due to chance. It is therefore more similar to a p value in statistics.

Here, you are not describing a single measurement. You are looking at the difference between two independent measurements. This is more like a T test. If you could achieve a five sigma level in that difference, that would give you overwhelming confidence that the difference is NOT due to chance.

You see the difference?

Yeah, that's the point, to rule out chance (with sufficient confidence) so that we can get on with purposefully exploring the 'but why?'.